1. Field of the Invention
The preferred embodiments are directed to probe devices for metrology instruments such as atomic force microscopes, and more specifically a method of producing a probe device using focused ion beam (FIB) etching, as well as a probe device produced thereby.
2. Discussion of the Prior Art
Several probe-based metrology instruments monitor the interaction between a cantilever-based probe and a sample to obtain information concerning one or more characteristics of the sample. For example, scanning probe microscopes (SPMs), including atomic force microscopes (AFMs), typically characterize the surface of a sample down to atomic dimensions by monitoring the interaction between the sample and a tip on the cantilever probe. By providing relative scanning movement between the tip and the sample, surface characteristic data can be acquired over a particular region of the sample, and a corresponding map of the sample can be generated.
The probe of the typical AFM includes a very small cantilever which is fixed to a support at its base and which has a sharp probe tip attached to the opposite, free end. The probe tip is brought very near to or into contact with a surface of a sample to be examined, and the deflection of the cantilever in response to the probe tip's interaction with the sample is measured with a deflection detector, such as an optical lever system, an example of which is described in Hansma et al. U.S. Pat. No. RE 34,489. The probe is scanned over a surface using a high-resolution three axis scanner acting on the sample support and/or the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography, elasticity, or some other surface property of the sample as described, e.g., in Hansma et al. U.S. Pat. No. RE 34,489; Elings et al. U.S. Pat. No. 5,266,801; and Elings et al. U.S. Pat. No. 5,412,980.
AFMs may be designed to operate in a variety of modes, including contact mode and oscillating mode. In contact mode operation, the microscope typically scans the tip across the surface of the sample while keeping the force of the tip on the surface of the sample generally constant. This effect is accomplished by moving either the sample or the probe assembly vertically to the surface of the sample in response to sensed deflection of the cantilever as the probe is scanned horizontally across the surface. In this way, the data associated with this vertical motion can be stored and then used to construct an image of the sample surface corresponding to the sample characteristic being measured, e.g., surface topography. Alternatively, some AFMs can at least selectively operate in an oscillation mode of operation such as TappingMode™ operation. (TappingMode is a trademark of Veeco Instruments, Inc.) In TappingMode™ operation the tip is oscillated, typically at or near a resonant frequency of the cantilever of the probe. The amplitude or phase of this oscillation is kept constant during scanning using feedback signals, which are generated in response to tip-sample interaction. As in contact mode, these feedback signals are then collected, stored, and used as data to characterize the sample.
Regardless of their mode of operation, AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers fabricated using photolithographic techniques. Because of their resolution and versatility, AFMs are particularly important measurement devices in many diverse fields including with particular application in connection with the present preferred embodiments semiconductor manufacturing.
A scanning probe microscope, such as an atomic force microscope (AFM) operates by providing relative scanning movement between a measuring probe and a sample while measuring one or more properties of the sample. A typical AFM system is shown schematically in FIG. 1. An AFM 10 employing a probe device 12 including a probe 14 having a cantilever 15 is coupled to an oscillating actuator or drive 16 that is used to drive probe 14, in this case, at or near the probe's resonant frequency. Commonly, an electronic signal is applied from an AC signal source 18 under control of an AFM controller 20 to cause actuator 16 to drive the probe 14 to oscillate, preferably at a free oscillation amplitude Ao. Probe 14 is typically actuated toward and away from sample 22 using a suitable actuator or scanner 24 controlled via feedback by controller 20. Notably, the actuator 16 may be coupled to the scanner 24 and probe 14 but may be formed integrally with the cantilever 15 of probe 14 as part of a self-actuated cantilever/probe. Moreover, though the actuator 24 is shown coupled to the probe 14, the actuator 24 may be employed to move sample 22 in three orthogonal directions as an XYZ actuator, i.e., both Z motion, and X-Y scanning motion such as in raster scanning.
Typically, a selected probe 14 is oscillated and brought into contact with sample 22 as sample characteristics are monitored by detecting changes in one or more characteristics of the oscillation of probe 14, as described above. In this regard, a deflection detection apparatus 17 is typically employed to direct a beam towards the backside of probe 14, the beam then being reflected towards a detector 26, such as a four quadrant photodetector. As the beam translates across detector 26, appropriate signals are transmitted to controller 20, which processes the signals to move actuator 24 in Z appropriately (and possibly indicate changes in the oscillation of probe 14). Commonly, controller 20 (such as an analog or digital P-I controller) generates control signals to maintain a constant force between the tip and sample, typically to maintain a setpoint characteristic of the oscillation of probe 14. For example, controller 20 is often used to maintain the oscillation amplitude at a setpoint value, AS, to insure a generally constant force between the tip and sample (by using actuator 24 to move either the probe or sample in Z). Alternatively, a setpoint phase or frequency may be used.
At present, the broadening use of SPM has demanded greater performance over a wider range of applications. For example, AFM metrology is increasingly being utilized in semiconductor fabrication facilities, primarily due to recent developments of automated AFM tools able to acquire sample measurements with higher throughput, such as the Dimension® line of AFMs offered by Veeco Instruments Inc. These tools are able to provide a variety of sub-nanoscale measurements, therefore making AFM a viable tool for measuring, for example, “critical dimensions” of device features such as trenches and vias.
One challenge in imaging semiconductor samples with an AFM is that such samples have features having large aspect ratios, for example, 50:1, and more, generally 10:1, and at least 3:1. Although the apex or distal end of the AFM probe device tip is typically nanometer-scale, it is often difficult or impossible to generate probe tip-sample interaction sufficient to reliably image features such as trenches and vias that have very high aspect ratios. In sum, given the mechanical interaction between the tip of the probe and the sample surface, performing measurements on such features can be particularly challenging with SPMs.
One solution is to form the tip of the probes with a similarly high aspect ratio (for example 20:1), shown in the prior art probe of FIG. 10. One known technique for producing such high aspect ratio probe devices includes using a process employing a focused ion beam (FIB) to mill away tip stock material. FIG. 10 illustrates an FIB milled tip. The resulting tip is on the order of 4 μm long with an aspect ratio of 25:1. In this case, the tip is milled from a semiconductor material that was initially microfabricated into a pyramid-shaped tip stock using conventional semiconductor fabrication techniques, including an anisotropic etch that yields the faceted (e.g., pyramid-shaped) tip stock.
FIG. 2A illustrates a schematic cross-section of a standard pyramid-shaped tip stock 34 formed to extend from a cantilever 32 of a probe device 30 according to a conventional technique described generally hereinafter. Starting with a wafer (e.g., silicon), tip stocks 34 associated with an array of probe devices are batch fabricated from the wafer for later FIB milling of the tips. About 350 to 450 probe devices typically will be included in the array, and in some situations, multiple wafers can be milled using a single machine. This process is typically performed with an anisotropic etch that yields tip stocks having the aforementioned pyramidal shape, including an apex 36 and a base 38. Once the tip stocks 34 are formed, ion beam milling can be performed to produce a high aspect ratio tip including a spike, for each probe device. The widths of the spike are about 80 and 170 nm respectively when measured at 2 μm and 4 μm height from the new apex on the FIB-milled tip. Each tip stock 34 is milled individually until the entire wafer is processed into probe devices 30.
With reference to FIG. 2B, one challenge with known FIB milling techniques is referencing the FIB source to apex 36 of the pyramid-shaped tip stock 34 when initiating the milling operation and the complications of “rubble” (see FIG. 2C) that always exists around the base. As noted earlier, this apex typically has a nanometer-sized dimension and its shape is highly variable, and thus is difficult to identify, even with sophisticated pattern recognition software (see FIG. 2B). When performing an FIB milling operation, if the source is not aligned with apex 36, more of tip stock 34 will typically need to be milled away to create the tip, and the shape of the resultant spike will not be optimal. More particularly, as the milling pattern is shifted relative to the original apex, the length of the resultant spike compared to the side lobes at each pyramid corner changes dramatically. In that case, the aspect ratio of the resultant tip may be less than theoretically possible because the apex of the stock (defining the point of the potentially largest height of the tip) is no longer present. Moreover, milling more material to produce the tip also requires a greater number of milling steps (described further below), and correspondingly more time to form the tips.
This alignment challenge also renders it even more difficult to produce a high aspect ratio tip considering that the tip is formed at an angle relative to the cantilever. This is so because, as understood in the art, the tip most often extends orthogonally to a plane substantially defined by the sample surface. Because the probe (i.e., cantilever) typically extends at an angle to the surface during AFM operation, the probe most often is fabricated to extend at an angle relative to the cantilever (typically 3° or 12°). With this background and noting that the complex pyramidal shape is hard to track as it is oriented at angle to the beam, a non-ideal tip often results (for example, milling may begin at point “a” which could result in milling too much of the tip stock, shown schematically in FIG. 3A, and discussed further below).
The pyramidal shape of the tip stock also complicates the FIB milling operation because large volumes of the sloped surfaces 35 extending from apex 36 to the base 38 of the tip stock 34 cannot be milled in one step; rather, a complex algorithm to control the FIB source, for example, to track the slope of the tip stock surfaces to be milled is required. More particularly, turning to FIG. 3A, in a typical FIB milling operation, an FIB source 39 is instructed to mill portions of a tip stock 34 by controlling the location, ion beam current, area, and dwell time of a ion beam “B.” Because the pyramid-shaped stock has sloped surfaces, the overhead focused ion beam must spend differing amounts of time at each point on the slope to yield a resultant flat surface around the spike, i.e., there is non-uniformity in the thickness or height of the tip stock from the base. Most typically, as a result, and as shown in FIG. 3A, the control algorithm directs the beam to mill individual cylinders, anywhere from 5 to 100 μm in diameter, of the stock at a particular location/power, thereby “whittling” the stock down. These steps are organized into rectangular or circular groups referred to as mask 40. This is done in a series of patterned steps to create, for example, the sharp barb shown in FIG. 10, and shown schematically in FIG. 3B. In sum, each mask 40 constitutes a relatively small portion of the entire milled volume of the probe tip 44 such that often hundreds of masks corresponding to FIB process steps that are required to mill the tip, steps that again are controlled by a complicated control algorithm that must be developed by the manufacturer of the probe. In the end, each tip associated with each probe can take as long as 5 or more minutes to produce.
In addition, the sharp barb 44, and particularly apex 46, operates as the “active portion” of the tip, i.e., the portion that operatively interacts with the sample during the imaging process. With reference to FIG. 3B, the active portion of barb 44 has a height “h1,” even though the barb has an actual height “h2.” As shown, the active portion of barb 44 is limited by the imperfect milling process of a pyramidal-shaped starting stock, leaving residual portions 42 having a height extending above the bottom or base of barb 44. It is the difference between h1 and h2, which depends greatly on the shape of the starting stock that makes milling with current techniques particularly difficult. In other words, more milling must generally be done with inconsistently shaped tip stocks to account for variation. In the end, laborious milling was required for fabricating this type of probe and an improvement was desired.
Notably, even if tips may be produced with an active region having the desired aspect ratio, to do so, known processes require that a large tip stock 34 must be formed to enable deep milling to allow corresponding production of high aspect ratio tips. This requirement typically adds to an already large number of FIB milling steps, and also requires that a large volume of tip stock material be removed from the tip. Not only does this add to the inefficiency of known FIB probe fabrication processes, it adds significant mass to the resultant tip, thereby limiting the speed at which the probe devices can operate given the corresponding limited operational resonant frequencies.
Overall, the process of forming the tip such as that shown in FIG. 10 requires a large stock material and a complex algorithm to control the FIB source to mill a high aspect ratio tip. Forming the tip of each probe can take as many as fifty or more individual process steps to ultimately mill a tip stock into a tip such as that shown in FIG. 10, a tip that typically has less than optimal performance characteristics for present high throughput applications, due at least in part to aspect ratio limitations caused by h1 vs. h2.
The field of scanning probe microscopy in general, and essentially critical dimension AFM (CD-AFM), including deep trench AFM (DT-AFM), was thus in need of a new process of forming a probe, preferably using FIB milling to yield a high aspect ratio tip, but doing so with a minimum number of process steps, and with a sufficiently small volume of milled stock material. Preferably, the tip stock would have a shape defining a known volume (rather than a random shape) and would have a relatively uniform height from distal end to base on a tip-to-tip basis. Ideally, FIB milled tips that can be readily recognized and FIB milled in substantially less than 5 minutes while maintaining a high aspect ratio tip was desired.